1. Field of the Invention
The present invention relates to a laser module installing a laser diode and a photodiode therein.
2. Related Prior Art
A laser module with a metal stem mounting a laser diode thereon is well known in the field. For example, a Japanese patent published as H07-058413 has disclosed such laser module with four lead terminals, one of which is directly connected to the metal stem and the other of terminals are electrically isolated from the stem. When two devices, the laser diode and the photodiode, are installed in the module with three terminals electrically isolated from the stem, two of electrodes, among the anode and cathode electrodes of the laser diode and those of the photodiode, must be commonly connected to one terminal. One of the cathode and anode electrodes of the photodiode and one of the cathode and anode electrodes of the laser diode must be commonly connected to each other.
In particular, when the laser diode in the module is driven in the differential-driving mode, not only the optical waveform output from the laser diode may be perturbed but also the radiation noise may be induced. This is because the modulation current leaks to the photodiode via the terminal to which the laser diode and the photodiode is commonly connected.
In the case that the module may provide five or more lead terminals, the subject above mentioned does not become any problem. However, a package with such many lead terminals is not popular and does not show any cost merit. A package with four lead terminals, one of which is directly connected to the stem, for a semiconductor device is typically used for some electrical and optical devices.
Therefore, one of objects of the present invention is to provide a laser module capable of suppressing the ripple in the frequency response of the optical output and reducing the induced noise with limited count of lead terminals.